Upgraded code with RK4 and fixed timestep but it hasn't been tested at all yet.
Physics engine for linear motion implemented by a major bug with get getMotionState() function of btRigidbody is causing an access violation and I can't figure out how to prevent the problem.
In the process of debugging RK4 physics step. getMotionState() still basically borks the whole thing, so I'm just printing to the console in an attempt to work out why the integration isn't working properly.